Firearm thumb support



FIG. 1 is a perspective view of a firearm thumb support.

FIG. 2 is another perspective view thereof.

FIG. 3 is a side view thereof; and,

FIG. 4 is a front view thereof.

The broken lines in the figures depict portions of the firearm thumb support that form no part of the claimed design. 

CLAIM The ornamental design for a firearm thumb support, as shown and described. 